Dynamic and non-contact measurement of displacement or of permittivity by use of a capacitive sensor

ABSTRACT

A method for dynamic and non-contact measurement of the displacement of a conductive substance with respect to a capacitive sensor formed by two parallel conductive plates, superimposed, electrically insulated from one another, and fed by a high frequency signal having a predetermined voltage, the capacitive sensor being connected to a device for detecting a current value. Also disclosed is a method for dynamic and non-contact measurement of the permittivity of a dielectric substance between a conductive part and a capacitive sensor of the above-mentioned type.

This application is a continuation, of application Ser. No. 07/732,493,filed Jul. 18, 1991, pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for dynamic and non-contactmeasurement of a displacement of a grounded conductive substance withrespect to a capacitive sensor and to a method for dynamic andnon-contact measurement of the permittivity of a dielectric substancebetween a grounded conductive part and a capacitive sensor. Theinvention relates more particularly to a method for dynamic andnon-contact measurement of a displacement which can be used favourablyto permanently measure with relative simplicity the axial displacementof a shaft of a rotating machine or to measure a fluid level in a tank,and also to a method for dynamic and non-contact measurement of thepermittivity of a dielectric substance which can be used favourably toalso measure a fluid level in a tank or continuously monitor withrelative simplicity a possible change in the composition of a substanceflowing through a conduit.

2. Description of the Prior Art

U.S. Pat. No. 4,675,670 granted to HYDRO-QUEBEC describes an apparatusand a method for dynamic and non-contact measurement of the distanceseparating the surface of a first part, that may be conductive or not,from the surface of a second conductive part closely spaced from thefirst part and grounded, such as the stator and rotor of an electricgenerator. The apparatus and method can be permanently used withoutsignificant modification or excessive congestion, while providingprecise and reliable results even in the presence of intense magneticfields or temperature variations.

The above-mentioned apparatus includes a sensor made of two parallelconductive plates, superimposed and electrically insulated from oneanother, and fed by a high frequency signal between 100 kHz and 10 MHzat a predetermined voltage between 5 and 100 volts, connected to adevice for detecting a current value, which is itself connected to adevice which processes the detected current value, such as a computer.

The sensor during its use forms a capacitor with the grounded conductivepart, so that the capacitance is determined by the following knownequation: ##EQU1## in which: K=εoεr, εo being the vacuum permittivity(8.854 pF/m) and εr being the relative permittivity of the dielectricsubstance between the nearest sensor plate from the conductive part andthis conductive part;

Ar is the overlapping surface of the conductive part on the sensorplate; and

D is the distance between the surface of the nearest sensor plate fromthe conductive part and this conductive part.

When the so formed capacitor is subjected to a high frequency signal, ameasureable current is induced in the sensor plates, of which theintensity responds in accordance with the following equation:

    i=ωCV                                                (2)

in which:

ω=2πf, f being the frequency of the emitted signal;

V is the voltage difference between the nearest sensor plate from theconductive part and this conductive part; and

C is the above-mentioned capacitance.

Equation (1) shows that for constant dielectric value K and overlappingsurface Ar, the capacitance C, and so the current i of equation (2),varies according to the inverse of the distance D separating the sensorfrom the conductive part, making possible the mentioned method fordynamic and non-contact measurement of the distance between the nearestcapacitive sensor plate from a conductive part and the conductive part.

As it can be easily seen, the apparatus can be similarly used to carryout the measurement of another variable parameter in equation (1), suchas the permittivity K or the overlapping surface Ar for instance, aslong as the other parameters are fixed.

SUMMARY OF THE INVENTION

A first object of the present invention is therefore to propose a newmethod using the above-mentioned apparatus for dynamic and non-contactmeasurement of the displacement of a grounded conductive substance withrespect to a capacitive sensor, this first method comprising the stepsof:

(a) positioning the capacitive sensor at a fixed distance close to theconductive substance, the plates of the capacitive sensor being parallelto the plane in which the conductive substance extends, such that adisplacement of this substance in the mentioned plane modifies anoverlapping surface formed by portions of the conductive substance andthe capacitive sensor which are superimposed;

(b) detecting the current induced by a high frequency signal in thecapacitive sensor, this current having a value varying in a directlyproportional relationship with the overlapping surface; and

(c) determining the value of the displacement of the conductivesubstance in respect with the capacitive sensor according to the valueof the current.

This method is not restricted to turbine machines or to tanks since itallows a displacement measurement of any conductive substance withrespect to the sensor, as long as the overlapping surface of theconductive substance on the sensor varies at the time of thedisplacement.

A second object of the present invention is to propose a new method alsousing the above-mentioned apparatus for performing a dynamic andnon-contact measurement of the permittivity of a dielectric substance itbetween a grounded conductive part and a capacitive sensor, this secondmethod comprising the steps of:

(a) positioning the capacitive sensor at a fixed distance close to theconductive part so that the dielectric substance whose permittivity isto be measured is between the conductive part and the capacitive sensor;

(b) detecting the current induced by a high frequency signal into thecapacitive sensor, this detected current varying in a directlyproportional relationship with the permittivity of the dielectricsubstance; and

(c) determining the value of the permittivity of the dielectricsubstance between the conductive part and the capacitive sensoraccording to the value of the current.

Once again, this method is not restricted to tanks or to the detectionof a change in the composition of a substance flowing in a conduit sinceit allows the permittivity of a dielectric substance between acapacitive sensor and any conductive part to be measured.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention as well as its numerous advantages will be betterunderstood by the following non-restrictive description of possibleembodiments made in reference to the appended drawings in which:

FIG. 1 shows a principle schematic of the methods according to theinvention;

FIG. 2 is a schematic diagram showing the first method according to theinvention for the displacement measurement of a conductive substancelarger than the capacitive sensor;

FIG. 3 shows a characteristic curve of the measured current value for anembodiment of the invention according to FIG. 2;

FIG. 4a shows the first method according to the invention for thedisplacement measurement of a coupling joint between a thermal turbineand an alternator;

FIG. 4b is a detailed side view of the coupling joint shown in FIG. 4a;

FIG. 4c is a detailed top view of the coupling joint shown in FIG. 4a;

FIG. 5 shows the first method according to the invention fordisplacement measurement using several sensors;

FIG. 6a shows the first method according to the invention for the levelmeasurement of a conductive or highly polar fluid in an electricallyconductive tank;

FIG. 6b shows the first method according to the invention for the levelmeasurement of a conductive or highly polar fluid in a tank made of anelectrically insulated material;

FIG. 6c shows the second method according to the invention for the levelmeasurement of non-conducting fluid in an electrically conductive tank;

FIG. 6d shows the second method according to the invention for the levelmeasurement of non-conducting fluid in a tank made of an electricallyinsulated material;

FIG. 7 shows the second method according to the invention for thedetection of a water contamination in oil flowing in a conduit;

FIG. 8 shows a diagram of the measurement, detection and calibrationcircuit used within the framework of the present invention; and

FIG. 9 is a block diagram showing the principal functions of theapparatus used by the methods according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1 of the accompanying drawings, the apparatus 1used to implement the methods according to the invention comprises atleast one capacitive sensor 3 made of two plates 5 and 7 which areconductive, superimposed and electrically insulated from one another byan appropriate insulator 9, a supply and detection device 11 to supplythe two plates 5 and 7 of each sensor with the same high frequencysignal at same voltage and to detect the value of the induced current inthe plate 7 by the high frequency signal, and a cable 13 connecting theplates 5 and 7 of each sensor to device 11. This device 11 generates anelectric signal S, in relation to the detected current, which may besent to a data processing apparatus such as a computer for example, tocarry out the measurement according to the methods of the invention.

The high frequency signal feeding the plates 5 and 7 of the sensor ispreferentially set at a frequency between 100 kHz and 10 MHz with avoltage between 5 and 100 volts peak-to-peak, in order to avoid possibleparasitic noise, return impedance or insulation problems. The effect ofparasitic noise can also be decreased by selecting the optimal dimensionof the plates 5 and 7 and their separation distance.

The processing apparatus takes into consideration the non-linearity dueto the construction and physical parameters of the capacitive sensor 3,so is calibrated beforehand to perform a precise conversion of themeasured current in relation to the parameter making the object of themeasurement using the methods according to the invention.

In the first method according to the invention (the method for dynamicand non-contact measurement of a displacement), the capacitive sensor 3is positioned by appropriate means at a fixed distance D close to theconductive substance 15 such that its plates 5 and 7 are parallel to theplane in which the conductive substance 15 extends. In this plane, thecommon surface of the conductive substance 15 and the capacitive sensor3 constitutes an overlapping surface 17. It is essential according tothe present method that the area of the overlapping surface 17 varies atthe time of a displacement of the conductive substance 15 with respectto the capacitive sensor 3 since the measured current is directlyproportional to this area and that this measured current makes preciselythe object of the measurement of the displacement.

FIG. 2 shows a use of the first method according to the invention (themethod for measurement of a displacement) where the conductive substance15 is larger than the capacitive sensor 3, the sensor 3 being ofrectangular shape. Although this shape is not essential, it provides inthe present use some characteristics of particular interest, which aregraphically represented in FIG. 3.

FIG. 3 shows the linearity resulting from the embodiment shown in FIG.2, between the displacement in the direction of arrow 19 of theconductive substance 15 with respect to the capacitive sensor 3 and theelectric signal S generated by the device 11. This characteristic isbased on the fact that the overlapping surface 17 linearly increases inrelation to the mentioned displacement. The curve 21 is mainly linear,although subjected to deflection 23 and 25 at each extremity when theedge 27 of the conductive substance 15 crosses the edge 29 of thecapacitive sensor 3.

As shown in FIG. 4a, the first method according to the invention (formeasurement of a displacement) may be used to measure the elongation ofa turning shaft 31 linking a thermal turbine 33 to an alternator 35. Thecoupling joint 37 acts as conductive substance and therefore has to begrounded beforehand. As shown in the enlargements 4b and 4c, the sensor3 is positioned parallel to and at a fixed distance D' from the couplingjoint 37, such as to measure a possible displacement of the joint 37 inthe direction of arrow 39, represented by the dotted lines 41. It isnecessary that the distance D" between the capacitive sensor 3 and theshaft 31 shall be long enough with respect to the distance D' so thatthe shaft 31 has a negligible capacitive effect beside the sensor 3 anddoes not affect the measurement of the displacement. Since the width B'of the capacitive sensor 3 is smaller than the width B" of the couplingjoint 37 and as a result of the fact that this sensor has a rectangularform, the relation between the displacement and the signal S is linear,reducing in this way the complexity of subsequent processings of thesignal S.

Referring now to FIG. 5, when the displacement of the conductivesubstance 15 in the direction of arrow 43 is too large to be measuredonly by one sensor, it is possible to use several capacitive sensors 3positioned according to the first method of the invention formeasurement of a displacement, such as the displacement of theconductive substance 15 alters the overlapping surface of at least onesensor, changing so the value of the measured current. An easy way toobtain this result is to linearly position the sensors one afteranother.

As shown in FIG. 6a, the first method according to the invention (formeasurement of a displacement) may be used to measure the level 45 of afluid 47 which is electrically conductive or highly polar, such as forexample water in particular conditions or mercury, in a tank 49 which isalso electrically conductive. In that case, the fluid 47 acts as theconductive substance and can be connected to the ground 51 using theexpedient of the tank 49 which itself is conductive. The capacitivesensor 3 may be positioned directly on the inner wall 53 of the tank 49simply by sticking or by appropriate means, and has to be inevitablycovered by an insulating and sealing protective layer. To allowmeasurement of any level 45 of fluid 47 in the tank 49, the sensor 3only has to be as long as the height of the wall 53 of the tank 49. Inthe case where the tank 49 is electrically insulated, as shown in FIG.6b, the electrically conductive or highly polar fluid 47 is connected tothe ground 51 by the help of a grounding secondary electrode 55, made ofa simple metallic plate for example, disposed at the bottom of the tank49 inside or outside.

Referring now to FIG. 6c, the apparatus 1 is now used to implement thesecond method according to the invention (the method for dynamic andnon-contact measurement of the permittivity of a dielectric substance)to measure the level 45 of an electrically non-conductive fluid 47 of arelative dielectric constant higher than the one of the air (1 unity) asthe one of oil, gasoline or other petroleum derivatives, for example. Inthis embodiment, the signal S of detected current has an offset whichcan be adequately corrected or compensated when the tank is empty. Thisoffset is all the more important as the dielectric constant of thesolution approaches that of air. A grounding secondary electrode 55 atleast as long as the sensor 3, made of a simple metallic plate forexample, is connected to the ground 51 and disposed close to thecapacitive sensor 3, itself disposed on the inner wall 53 of theelectrically conductive tank 49. The protection of the sensor 3 is notnecessary but desirable. In the case where the tank 49 is electricallyinsulated, as shown in FIG. 6d, the sensor 3 can be disposed on theinner or outer wall of the tank 49.

In FIG. 7, the second method according to the invention is used todetect the composition of a substance 57, possibly altered by acontamination inside an electrically insulated conduit 59. The sensor 3is positioned directly on the wall of the conduit 59 in a way that thesubstance 57 inevitably passes between an electrode 61 connected to theground 63 and this sensor 3, changing the permittivity previouslymeasured when a contamination occurs. Knowing the value of thepermittivity of the substance 57 or proceeding to tests for determiningit, it would be possible to connect the supply and detection device 11to an alarm system adjusted beforehand to report any occurring change inthe composition of the substance 57 when it is detected.

FIG. 8 shows a diagram of the circuit of the supply and detection device11 of the apparatus 1 used according to the invention. In this circuit,there is a high frequency signal generator 65. This generator suppliesthe plate 5 of the sensor 3 which forms a fixed capacitance Cf with theambient. The plate 7 of the sensor 3 forms, with the surface of theconductive substance 15, a variable capacitance Cv. There is also aparasitic capacitance particular to each type of sensor. This parasiticcapacitance is indicated by the symbol Cp. A current detector 67comprises a low value impedance 69 mounted in series between thegenerator 65 and the plate 7 of the sensor. The current detector 67 alsocomprises an insulating circuit connected to the impedance 69 to measureisolately the high frequency voltage signal across this impedance 69 andto extract from the signal so measured a signal proportional to themeasured current. The circuit can also comprise an isolation transformer71 connected, on a side, in parallel to the impedance 69 and, on theother side, to an adjustable gain amplifier 73. This amplifier is at itsturn connected to an amplitude demodulator of known type 75 whichreceives the amplifier signal via a filter 77. The demodulator 75outputs the requested signal proportional to the detected current andtransmits it to appropriate processing means 89 described in furtherdetail hereinafter. For an automatic correction of the apparatus drift,the detector 67 may also incorporate calibration means 79 to replacemomentarily the variable capacitance Cv. These calibration means 79,connectable by way of a switch 81, are made of two impedances 83 and 85of known value allowing to obtain two reference signals (high and lowcalibrations). These impedances 83 and 85 are switched by a relay 87.

It should be mentioned that the use of such calibration means 79 isoptional. In some cases, it allows, before each data acquisition, totake account the amplifier drifts as well as the generator drifts, andfurther to allow an easy determination of the gain and offsets.

As shown in FIG. 9, the processing means 89 are connected to the currentdetector 67.

According to a first example of embodiment, these processing means 89can be built with a processing circuit including a microprocessor 91whose function is to assure a processing and a recording of the currentsignals detected by the current detector 67 until they are required.When the information is required, the signals may be then transmitted bythe microprocessor 91 to a computer 93 which can be equipped withexternal recording means 95 (magnetic disks, etc.) and output means 97(printer, etc.). In the case where several sensors 3 are used, thecomputer 93 can be connected to a microprocessor 91 associated to eachsensor 3 in way to individually process the memorized signal in each ofthe microprocessors 91 associated with each of the sensors 3. Enlargedlines illustrated in FIG. 9 show, by way of an example, possibleconnections to other microprocessors 91. The introduction of data whichprovide the current-displacement or current-permittivity relation foreach type of sensor is done once for each type of sensor in thelaboratory. As previously indicated, it may be necessary to make acalibration prior to each data acquisition to take account of the driftsof the amplifier and the generator.

According to another example of embodiment, processing means 89 can bebuilt with an alarm circuit 99. This alarm circuit 99 can be installedinstead of the processing circuit or jointly with it. The goal to beachieved by this alarm circuit 99 is to generate immediately an alarmsignal if the value of the detected current corresponds to apredetermined critical displacement or permittivity. The use of such analarm circuit 99, which may be connected easily and permanently, is veryadvantageous mainly in the case of continuous and permanent monitoringof a rotative machine, tank level or contamination of a substance insidea conduit.

What is claimed is:
 1. Method for dynamic and non-contact measurement ofa displacement of a grounded conductive substance with respect to acapacitive sensor formed of two parallel conductive plates,superimposed, electrically insulated one from the other, and fed by ahigh frequency signal at a predetermined voltage originating from asignal generator, said capacitive sensor being connected to a device fordetecting a current value, said method comprising the steps of:(a)positioning said capacitive sensor close to and at a perpendicular fixeddistance from a plane in which said conductive substance extends, saidplates being substantially parallel to said plane, and displacing saidconductive substance in said plane to modify an overlapping surfaceformed by portions of said conductive substance and said capacitivesensor which are superimposed; (b) detecting a current induced by saidhigh frequency signal in said capacitive sensor, said current having avalue varying in a directly proportional relationship with saidoverlapping surface; and (c) determining the value of the displacementof said conductive substance with respect to said capacitive sensoraccording to the value of said current.
 2. Method according to claim 1,wherein the detection of said current is carried out by an insulatingcircuit connected in parallel to a low impedance mounted in seriesbetween said signal generator and the nearest plate of the sensor fromthe conductive substance, said insulating circuit taking measurement ofsaid high frequency signal at terminals of said low impedance, and byextracting from said measurement a signal proportional to said current.3. Method according to claim 1, wherein determination of the value ofsaid displacement in relation to the value of said current is carriedout by means of electronic devices designed or programmed in function ofvarious physical parameters of said capacitive sensor, for determiningthe real value of said displacement corresponding to the value of saidcurrent by taking account of parasitic effects which are specific tosaid capacitive sensor and may affect the value of said current someasured.
 4. Method according to claim 1, wherein several capacitivesensors are positioned at step (a) such that the displacement of saidconductive substance modifies said overlapping surface on at least oneof said capacitive sensors.
 5. Method according to claim 1, wherein:saidconductive substance is the shaft of a rotative machine, thedisplacement subjected to said measurement being the one of a couplingjoint of said shaft in respect with said capacitive sensor.
 6. Methodfor dynamic and non-contact measurement of permittivity of a dielectricsubstance between a grounded conductive part and a capacitive sensorformed of two parallel conductive plates, superimposed, electricallyinsulated one from the other, and fed by a high frequency signal at apredetermined voltage originating from a signal generator, saidcapacitive sensor being connected to a device for detecting a currentvalue, said method comprising the steps of:(a) positioning saidcapacitive sensor close to and at a perpendicular fixed distance from asurface of said conductive part so that said dielectric substance whosepermittivity is to be measured is between said surface of saidconductive part and said capacitive sensor; (b) detecting a currentinduced by said high frequency signal in said capacitive sensor, saidcurrent varying in a directly proportional relationship with thepermittivity of said dielectric substance; and (c) determining the valueof the permittivity of said dielectric substance between said conductivepart and said capacitive sensor according to the value of said current.7. Method according to claim 6, wherein the detection of current iscarried out by an insulating circuit connected in parallel to a lowimpedance mounted in series between said signal generator and thenearest plate of the sensor from the conductive substance, saidinsulating circuit taking measurement of said high frequency signal atterminals of said low impedance, and by extracting from said measurementa signal proportional to said current.
 8. Method according to claim 6,wherein determination of the value of said permittivity in relation tothe value of the current is carried out by means of electronic devicesdesigned or programmed in function of various physical parameters ofsaid capacitive sensor, for determining the real value of saidpermittivity corresponding to the value of the current by taking accountof parasitic effects which are specific to said capacitive sensor andmay affect the value of said current so measured.
 9. Method according toclaim 6, wherein several capacitive sensors are positioned in step (a)in a way that the permittivity of said dielectric substance may bedetermined in several places.